Irish Twins

Originally I had heard (from a much less reliable source) that Irish twins were siblings born within two years of each other. Well, sorry to let you guys down. No Irish twins here :o)! Thought this was interesting anyway...

IRISH TWINS
[Q] From Swami Murugananda: “I recently heard the expression, Irish twins. What does it mean and would you cast some light as to its origin?”
[A] What it means is easy enough to explain. It refers to siblings born in the same calendar year, or otherwise less than twelve months apart. It’s clearly a deeply derogatory comment about the stereotypical fecundity (and lack of contraception) of Irish Catholic families. It’s probably twentieth-century, but I can find little evidence that would help to tie it down (it may be relevant that it isn’t listed in the 1984 edition of Eric Partridge’s Dictionary of Slang and Unconventional English). So far as I can tell from the places I’ve found it, all online, it’s primarily an American expression; it’s also known in Britain, but it doesn’t often find its way into print, no doubt because it is considered offensive.


So, which is harder to handle: biological twins, Irish twins, or siblings born within 15 months of each other?